1. Field of the Invention
The present invention is directed to an X-ray examination device of the type having a C-arm which carries an X-ray radiator and a radiation receiver at its opposite ends, respectively.
2. Description of the Prior Art
Stationary and mobile X-ray examination devices are known which have a C-arm at whose ends an X-ray radiator and a radiation receiver are arranged, respectively. The C-arm is shiftable along its circumference in a circumferential direction and is rotatable around a horizontal axis, so that transirradiation of a patient oriented on a table is possible at different directions. The table is usually adjustable in three dimensions.
In such X-ray examination devices for the transillumination of a patient, it is problematic to arrange the X-ray radiator and the radiation receiver so that the patient's body region to be examined lies in the imaging field of the radiation receiver. German OS 295 09 546 (corresponding to United Kingdom Published Application 2 302 492) thus teaches the use of a laser light-beam localizer formerly two line lasers for the purpose of projecting a target cross onto the body surface of a patient. The line lasers are arranged at the X-ray radiator or the radiation receiver so that the center beam of an X-ray bundle emanating form the X-ray radiator proceeds along the lines of intersection of the two laser beams of the line lasers.
German OS 195 24 951 (corresponding to U.S. Pat. No. 5,690,107) teaches a computed tomography apparatus with a laser means for a similar purpose wherein, for example, in the preliminary stage of a tumor treatment of a patient by means of therapeutic radiation, the position and the extent of the tumor in the patient are detected in a diagnostic image produced with the computed tomography apparatus. Once the tumor's position and size are thus determined, a projection with the aid of line lasers of the laser means is made onto the body surface of the patient for marking, in order to enable a reproducible positioning of the patient relative to the therapeutic radiation means.
For simplifying the examination of a patient, in a C-arm X-ray device of the type previously described, an isocenter can be identified which is characterized by the center beam of an X-ray bundle emanating form the X-ray radiator always running through the center of rotation of the C-arm, so that different projections can be performed without a repositioning of the patient relative to the C-arm. If the subject of interest in the patient is in the isocenter before a shifting of the C-arm, then this subject does not wander out of the isocenter given an orbital motion and given an angling motion of the C-arm, since this isocenter always remains in the same point in space, independently of the orbital and angling motions, as long as the C-arm is not shifted vertically or horizontally. In mobile X-ray examination devices, however, the table generally is independent of the C-arm device. Even if, given shifting of the C-arm, the isocenter of the mobile C-arm device does not change with reference to its spatial position, the examination object can wander out of the imaging area if the table position changes. It is therefore desirable to be able to check the spatial position of the isocenter.
German PS 40 03 350 teaches a C-arm device with an isocentrically shiftable C-arm which is provided with two lasers whose beams cross in the isocenter of the C-arm and thus characterize the position of the isocenter.
It has proved to be disadvantageous, however, to position a patient relative to the C-arm device in this way, because the isocenter is located in the body of the patient. Two laser points are projected onto the body surface of the patient from different directive without a visible intersection, and thus the position of the isocenter is not clearly identifiable.
Beyond this, the lasers are arranged at the C-arm in such a way that the laser points wander--particularly around the angling axis of the C-arm--given angling movements of the C-arm over the body surface of the patient, so that precise identification of the spatial position of the isocenter is further impeded.